As examples of prior art for providing stiffness, which are applied to joints of a robot manipulator, there is one approach that uses a mechanical spring. Further, there is another approach that detects forces exerted on a manipulator and controls a drive motor of a joint accordingly to accomplish the effect of virtual spring without any additional mechanism.
A document entitled “The second IARP-IEEE/RAS Joint Workshop on Technical Challenge for Dependable Robots in Human Environment” (KOREA INSTITUTE OF SCIENCE AND TECHNOLOGY, pp. 88-95, 2002) discloses “Safe arm design for service robot” (Kim). The arm design disclosed in such a document enhances safety of a manipulator by additionally mounting a mechanical spring to a joint of a robot. It is configured such that linear springs are disposed in a rotation direction so as to function like rotation springs. However, it operates only with determined spring constant and controlling its stiffness according to its operation is impossible.
Further, another document entitled “The IEEE/RSJ International Conference on Intelligent Robots and Systems” (pp. 508-412, 1995) discloses “Development of one-D.O.F robot arm equipped with mechanical impedance adjuster” (Morita and Sugano). The robot arm disclosed in such a document is configured to adjust a length of a plate spring by changing a stationary point (center of motion) of the plate spring along a linear guide by means of a separate drive mechanism to thereby control stiffness. In said robot arm, however, although variable stiffness is possible according to each state, additional mechanism must be driven for its operation. Thus, there is a problem in that stiffness cannot be rapidly varied.
Furthermore, many attempts have been made with regard to approaches for controlling a joint to show the effect of virtual spring by using a force sensor. As one example, a document entitled “The Intl. Conf. On Robotics and Automations” (pp. 1710-1716, 1002.) discloses “DLR's torque-controlled light weight robot iii” (Hirzinger). The light weight robot disclosed in such a document is configured to mount a sensor, which is capable of measuring force or torque, to an end portion of a robot manipulator or a joint portion of a link, while controlling a controller to show the effect of virtual spring based on inputted sensor information. In said light weight robot, however, although the spring effect is shown without any separate mechanism, there is no measure or remedy in case of an electrical breakdown. Further, there is a problem in that efficiency and performance are low since position control and force control are carried out by a single controller.